This or that for boy & girl?!?!

Hi, we are expecting in November and don't know yet if it's a boy or a girl. We are torn between two names for each gender. We already have two kids, Caleb, who is 4. And, Emmi, who is 2 1/2. If it's another girl, we want another nicknamey name to go with Emmi. Our top two choice are Molly and Elle (pronounced "L"). For boys, our top two choice are Luke and Ben (not Benjamin). We are name them what you will call them people! Which do you like best for each? Also, any middle name ideas for Molly? For Elle, we are thinking of Elle Charlotte. Thanks!